1. Field of the Invention
The present invention relates to an apparatus and method for polishing workpiece, and more particularly to an apparatus and method for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. The photolithographic process requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable having a polishing cloth attached thereon, and a top ring for applying a constant pressure on the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable, and the surface of the semiconductor wafer on which circuits are formed is chemically and mechanically polished, while supplying a polishing liquid onto the polishing cloth. This process is called chemical mechanical polishing (CMP).
The polishing apparatus is required to have such performance that the surfaces of semiconductor wafers have a highly accurate flatness. Therefore, it is considered that the holding surface, i.e. the lower end surface of the top ring which holds a semiconductor wafer, and the upper surface of the polishing cloth which is held in contact with the semiconductor wafer, and hence the surface of the turntable to which the polishing cloth is attached, preferably have a highly accurate flatness, and the holding surface and the surface of the turntable which are highly accurately flat have been used. It is also considered that the lower surface of the top ring and the upper surface of the turntable are preferably parallel to each other, and such parallel surfaces have been used.
It is known that the polishing action of the polishing apparatus is affected not only by the configurations of the holding surface of the top ring and the contact surface of the polishing cloth, but also by the relative velocity between the polishing cloth and the semiconductor wafer, the distribution of pressure applied to the surface of the semiconductor wafer which is being polished, the amount of the polishing liquid on the polishing cloth, and the period of time when the polishing cloth has been used. It is considered that the surface of the semiconductor wafer can be highly accurately flat if the above factors which affect the polishing action of the polishing apparatus are equalized over the entire surface of the semiconductor wafer to be polished.
However, some of the above factors can easily be equalized over the entire surface of the semiconductor wafer, but the other factors cannot be equalized. For example, the relative velocity between the polishing cloth and the semiconductor wafer can easily be equalized by rotating the turntable and the top ring at the same rotational speed and in the same direction. However, it is difficult to equalize the amount of the polishing liquid on the polishing cloth because of a centrifugal forces imposed on the polishing liquid.
The above approach which tries to equalize all the factors affecting the polishing action, including the flatnesses of the lower end surface, i.e. the holding surface of the top ring and the upper surface of the polishing cloth on the turntable, over the entire surface of the semiconductor wafer to be polished poses limitations on efforts to make the polished surface of the semiconductor wafer flat, often resulting in a failure to accomplish a desired degree of flatness of the polishing surface. According to the study of the inventors, it is found that the lower surface (holding surface) of the top ring and the polishing surface of the turntable are preferably not parallel and flat.
It is therefore an object of the present invention to provide an apparatus and method for polishing a workpiece which can easily correct irregularities of a polishing action on a workpiece such as a semiconductor wafer, and polish a workpiece with an intensive polishing action or a weak polishing action on a desired localized area thereof.
In order to achieve the above object, according to a first aspect of the present invention, there is provided an apparatus for polishing a workpiece, the apparatus comprising: a turntable having a polishing surface; a top ring for holding a workpiece and pressing the workpiece against the polishing surface; a holding surface of the top ring for holding the workpiece, the holding surface being deformable by fluid having variable pressure; and a retainer ring for retaining the workpiece within the holding surface of the top ring, the retainer ring pressing the polishing surface under a variable pressing force.
According to another aspect of the present invention, there is provided a method for polishing a workpiece, the method comprising: holding a workpiece between a polishing surface of a turntable and a holding surface of a top ring; pressing the workpiece against the polishing surface in such a state that the holding surface for holding the workpiece is deformed to a desired shape by fluid having variable pressure; and pressing a retainer ring for retaining the workpiece within the holding surface of the top ring against the polishing surface under a variable pressing force.
FIG. 1 shows the basic principles of the first aspect of present invention. As shown in FIG. 1, the top ring 1 comprises a top ring body 2, and a holding plate 3 for holding a workpiece, to be polished, such as a semiconductor wafer 4. A chamber C is defined between the top ring body 2 and the holding plate 3, and is connected to a fluid source 5 through a regulator R1. An elastic pad 6 of polyurethane or the like is attached to the lower surface of the holding plate 3. A retainer ring (guide ring) 7 for holding the semiconductor wafer 4 on the lower surface, i.e. the wafer holding surface 3a of the holding plate 3 is disposed around the outer peripheral portion of the top ring 1. A fluid pressure bag 8 comprising an annular tube is provided between the retainer ring 7 and the top ring 1. The fluid pressure bag 8 is connected to the fluid source 5 through a regulator R2. A turntable 22 having a polishing cloth 21 attached thereon is disposed below the top ring 1. The polishing cloth 21 constitutes a polishing surface which is brought in sliding contact with the semiconductor wafer 4 for thereby polishing the semiconductor wafer 4.
The top ring 1 is connected to a top ring shaft 12 through a ball 11. The top ring shaft 12 is connected to a fluid pressure cylinder 14 fixedly mounted on a top ring head 13. The fluid pressure cylinder 14 serves as an actuator for moving the top ring 1 vertically, and is connected to the fluid source 5 through a regulator R3.
In the above structure, by supplying a pressurized fluid such as a compressed air to the fluid pressure cylinder 14 from the fluid source 5, the top ring 1 presses the semiconductor wafer 4 to be polished against the polishing cloth 21 on the turntable 22 under a certain pressing force F1 for thereby polishing the semiconductor wafer 4. The pressing force F1 is variable by regulating the regulator R3.
FIG. 2 is a schematic view showing the configuration of the wafer holding surface 3a of the holding plate 3. In FIG. 2, the horizontal axis represents a distance (mm) from the center (O) of the holding plate 3, and the vertical axis represents a height of the wafer holding surface. In FIG. 2, alternate long and short dash line xe2x80x9cdxe2x80x9d shows the condition that the wafer holding surface 3a is flat. In this condition, no pressurized fluid is supplied to the chamber C, and the polishing pressure is not applied to the wafer holding surface 3a while polishing is not performed. During polishing, when a pressurized fluid such as a compressed air is supplied to the chamber C from the fluid source 5, the wafer holding surface 3a of the holding plate 3 is curved by a pressing force of the pressurized fluid in a convex shape in a downward direction as shown by the curve xe2x80x9caxe2x80x9d in FIG. 2. That is, the wafer holding surface 3a defines a convex spherical surface. In this condition, the central portion of the semiconductor wafer 4 is pressed by the downwardly convex holding plate 3 against the polishing cloth 21 in a pressure higher than that applied onto the outer circumferential portion thereof. Thus, if the amount of a material removed from the outer circumferential portion of the semiconductor wafer 4 is larger than the amount of a material removed from the central portion of the semiconductor wafer 4, insufficient polishing action at the central portion of the semiconductor wafer can be corrected by utilizing deformation of the holding plate 3 caused by the pressurized fluid.
On the other hand, if the amount of a material removed from the central portion of the semiconductor wafer 4 is larger than the amount of a material removed from the outer circumferential portion of the semiconductor wafer 4, the regulator R1 is controlled to reduce the pressure of the pressurized fluid supplied from the fluid source 5 to the chamber C or to stop the supply of the pressurized fluid to the chamber C, thereby making the wafer holding surface 3a of the holding plate 3 in the form of curve xe2x80x9cbxe2x80x9d or xe2x80x9ccxe2x80x9d shown in FIG. 2. Therefore, the polishing pressure applied to the central portion of the semiconductor wafer 4 is decreased and the polishing pressure applied to the outer circumferential portion of the semiconductor wafer 4 is increased, in comparison with the condition caused by the curve xe2x80x9caxe2x80x9d. Thus, insufficient polishing action at the outer circumferential portion of the semiconductor wafer can be corrected, and the entire surface of the semiconductor wafer 4 can be uniformly polished.
When the supply of the pressurized fluid to the chamber C is stopped, the wafer holding surface 3a is curved due to a polishing pressure in a slightly convex shape in an upward direction as shown by the curve xe2x80x9ccxe2x80x9d. That is, the wafer holding surface 3a defines a concave spherical surface. If it is desirable to cause the wafer holding surface 3a of the holding plate 3 to curve upwardly in a higher degree than the condition shown by the curve xe2x80x9ccxe2x80x9d, the chamber C may be evacuated by the fluid source 5 comprising a vacuum pump. The shape or configuration of the wafer holding surface 3a can be made downwardly convex (convex spherical surface) or upwardly convex (concave spherical surface) or flat by developing positive pressure (pressure higher than atmospheric pressure) or negative pressure (pressure lower than atmospheric pressure) in the chamber C. The wafer holding surface 3a of the holding plate 3 can be deformed in a desired shape by selecting material and thickness of the holding plate 3. Preferred materials to be selected for the holding plate are, in consideration of the environments in which the polishing apparatus is used, corrosion-resistant and elastic materials, for example, austenitic stainless steel (SUS 304, SUS 316, etc.), aluminium titan, or resin material such as polyphenylene sulfide (PPS) or polyethelethelketone (PEEK). Preferred thickness of the holding plate is, in consideration of the safety against the interior pressure of the chamber (preferably, not more than 0.1 MPa), in the range of 3 to 8 mm, and preferably about 5 mm in case of austenitic stainless steel. In case of other materials, the thickness should be selected on the basis of modulus of elasticity, while taking into consideration the safety.
In parallel with correcting the shape of the wafer holding surface 3a of the top ring 1, the retainer ring 7 presses the polishing cloth 21 under a pressing force F2 by supplying a pressurized fluid such as a compressed air to the fluid pressure bag 8 from the fluid source 5.
In the present invention, the pressing force F1 exerted by the top ring 1 for pressing the semiconductor wafer 4 against the polishing cloth 21 on the turntable 22 is variable, and the pressing force F2 for pressing the retainer ring 7 against the polishing cloth 21 is also variable. These pressing forces F1, F2 are variable independently of each other. Therefore, the pressing force F2 which is applied to the polishing cloth 21 by the retainer ring 7 can be changed depending on the pressing force F1 which is applied by the top ring 1 to press the semiconductor wafer 4 against the polishing cloth 21.
Theoretically, if the pressing force F1 which is applied by the top ring 1 to press the semiconductor wafer 4 against the polishing cloth 21 is equal to the pressing force F2 which is applied to the polishing cloth 21 by the retainer ring 7, then the distribution of applied polishing pressures, which result from a combination of the pressing forces F1, F2, is continuous and uniform from the center of the semiconductor wafer 4 to its peripheral edge and further to an outer circumferential edge of the retainer ring 7 disposed around the semiconductor wafer 4. Accordingly, the peripheral portion of the semiconductor wafer 4 is prevented from being polished excessively or insufficiently.
FIGS. 3A through 3C schematically show how the polishing cloth 21 behaves when the relationship between the pressing force F1 and the pressing force F2 is varied. In FIG. 3A, the pressing force F1 is larger than the pressing force F2 (F1 greater than F2). In FIG. 3B, the pressing force F1 is nearly equal to the pressing force F2 (F1≈F2). In FIG. 3C, the pressing force F1 is smaller than the pressing force F2 (F1 less than F2 ) .
As shown in FIGS. 3A through 3C, when the pressing force F2 applied to the polishing cloth 21 by the retainer ring 7 is progressively increased, the polishing cloth 21 pressed by the retainer ring 7 is progressively compressed, thus progressively changing its state of contact with the peripheral portion of the semiconductor wafer 4, i.e., progressively reducing its area of contact with the peripheral portion of the semiconductor wafer 4. Therefore, when the relationship between the pressing force F1 and the pressing force F2 is changed in various patterns, the distribution of polishing pressures on the semiconductor wafer 4 over its peripheral portion and inner region is also changed in various patterns.
As shown in FIG. 3A, when the pressing force F1 is larger than the pressing force F2 (F1 greater than F2), the polishing pressure applied to the peripheral portion of the semiconductor wafer 4 is larger than the polishing pressure applied to the inner region of the semiconductor wafer 4, so that the amount of a material removed from the peripheral portion of the semiconductor wafer 4 is larger than the amount of a material removed from the inner region of the semiconductor wafer 4 while the semiconductor wafer 4 is being polished.
As shown in FIG. 3B, when the pressing force F1 is substantially equal to the pressing force F2 (F1≈F2), the distribution of polishing pressures is continuous and uniform from the center of the semiconductor wafer 4 to its peripheral edge and further to the outer circumferential edge of the retainer ring 7, so that the amount of a material removed from the semiconductor wafer 4 is uniform from the peripheral edge to the inner region of the semiconductor wafer 4 while the semiconductor wafer 4 is being polished.
As shown in FIG. 3C, when the pressing force F1 is smaller than the pressing force F2 (F1 less than F2), the polishing pressure applied to the peripheral portion of the semiconductor wafer 4 is smaller than the polishing pressure applied to the inner region of the semiconductor wafer 4, so that the amount of a material removed from the peripheral edge of the semiconductor wafer 4 is smaller than the amount of a material removed from the inner region of the semiconductor wafer 4 while the semiconductor wafer 4 is being polished.
As described above, according to the present invention, fluid is supplied to the upper surface opposite to the wafer holding surface 3a of the holding plate 3 of the rop ring 1, and, at this time, the pressure of the fluid is properly selected in the range of positive pressure to negative pressure to thereby make the shape of the wafer holding surface 3a downwardly convex or upwardly convex. In this connection, the semiconductor wafer 4 can be polished differently by varying the pressing force for pressing the semiconductor wafer 4 against the polishing cloth 21 at the outer circumferential portion and the central portion thereof. In some cases, the semiconductor wafer 4 is polished under the condition that the wafer holding surface 3a of the holding plate 3 is made flat.
In parallel with the above process, the pressing force F2 of the retainer ring 7 disposed around the top ring 1 is determined on the basis of the pressing force F1 of the top ring 1, and the polishing is performed while the retainer ring 7 presses the polishing cloth 21 under the determined pressing force F2. That is, the polishing operation of the semiconductor wafer 4 is performed under the shape correcting effect of the wafer holding surface 3a by fluid having positive pressure or negative pressure as well as the shape correcting effect of the polishing cloth 21 by the retainer ring 7. Thus, irregularities of the polishing action can be sufficiently corrected and the localized area (for example, the central portion, the outer circumferential portion) of the semiconductor wafer 4 is prevented from being polished excessively or insufficiently.
Acording to a second aspect of the present invention, there is provided an apparatus for polishing a workpiece, the apparatus comprising: a turntable having a polishing surface; a top ring for holding a workpiece and pressing the workpiece against the polishing surface; a pressing surface of the top ring for pressing the workpiece, the pressing surface being deformable by fluid having variable pressure; a fluid pressure bag provided between the pressing surface and the workpiece; and a retainer ring for retaining the workpiece within the top ring, the retainer ring pressing the polishing surface under a variable pressing force.
According to another aspect of the present invention, there is provided a method for polishing a workpiece, the method comprising: holding a workpiece between a polishing surface of a turntable and a pressing surface of a top ring; pressing the workpiece against the polishing surface through a fluid pressure bag provided between the pressing surface and the workpiece in such a state that the pressing surface for pressing the workpiece is deformed to a desired shape by fluid having variable pressure; and pressing a retainer ring for retaining the workpiece within the top ring against the polishing surface under a variable pressing force.
According to the second aspect of the present invention, the top ring has characteristics of the top ring of diaphragm-type, defined in the first aspect of the present invention, having a structure in which a holding surface for holding the workpiece is deformable by fluid pressure, i.e. controllability of the distribution of pressure on the surface to be polished as well as characteristics of the top ring of membrane-type, disclosed in, for example, Japanese laid-open patent publication No. 5-69310, having a structure in which a membrane is provided, i.e. applicability of uniform pressure onto the backside of the workpiece. Specifically, this top ring can apply controlled pressure partially onto the outer circumferential portion or the central portion of the workpiece, and apply uniform pressure onto the entire surface of other portion. Further, this top ring can control the region (width) of the outer circumferential portion or the central portion of the workpiece to which pressure is applied.
According to another aspect of the present invention, there is provided an apparatus for polishing a workpiece, the apparatus comprising: a turntable having a polishing surface; a top ring for holding a workpiece and pressing the workpiece against the polishing surface; a holding surface of the top ring for holding the workpiece; a retainer ring for retaining the workpiece within the holding surface of the top ring; and a pressing mechanism for pressing the retainer ring against the pressing surface under a variable pressing force, the pressing mechanism comprising a ring member vertically movably provided, and a space for allowing fluid having variable pressure to be supplied to press the ring member against the retainer ring.
With the above arrangement, even if the retainer ring is worn, the pressing mechanism for pressing the retainer ring can press the retainer ring against the polishing surface under a desired pressing force.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.